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Three tiers in different projects

I see in some projects tree tiers classes in single project (in app_code) and access them. But in some other projects there are different projects for all lears in one solution. Which is the better approch and why??? Thanks in advace.

Re: Three tiers in different projects

The layers reside on the same machine where the tiers can reside on different machines.

Layer communicates with each other either by Value or by Reference. However the Tiers could be on different machines, so they communicate by Value only – as serialized objects.

Layer is logical separation of code and Tier is physical separation of code, In other words we can say Tiers are the physical deployment of layers. Or a Tier becomes a Layer if it could be physically separated from the
Layers it consumes.

N-Tier Architecture is a physical structuring, while
a N-Layer Architecture is a logical structuring.

Now coming to 3 Layer Architecture and
3 Tier Architecture after getting an idea about Layer and Tier.

In 3 layer architecture, the Database Access Layer (DAL),
Business Logic Layer (BLL) and User Interface Layer (UIL) resides as 3 different project and the output of these 3 projects (.dll file) must be together in the
same server or on same machine in order for the system to run.However in 3 tier architecture, the
Database Access Layer (DAL), Business Logic Layer (BLL)and User Interface Layer (UIL) reside as 3 different projects. But each of the projects can be deployed at the
different server or at the different machines and distributed functionality is explored.

In 3 Layer we have distinct namespaces and classes for the various layes i.e. we’re only talking about
logical organization of code and passing objects between layers is easy as all assemblies will run under same application in single machine.But 3 tier applications are not straight as 3 layers, because each layer will be deployed
in different machines. So here, we need a mechanism to send the objects between different machines over the network hence we use technologies called
Web Services, Remoting or WCF,
etc.in order to implement 3 tier applications

3 Layer Architecture will improve readability and
reusability, Minimizes the application changes due to impact of the changes in other layers. While 3 Tier Architecture has all advantages of 3 layer +
scalability as application will be deployed in different machines so load will be shared among the tiers and scalability will increase.